The present invention relates to movement sensors and in particular to micromechanical rotational rate gyroscopes using the Coriolis force.
Micromechanical Coriolis-force rotational rate gyroscopes have many fields of application, for example the determination of the position of a motor vehicle or aircraft. Such devices or sensors in general have a movable mechanical structure that is excited to perform a periodic oscillation. This periodic oscillation induced by excitation is referred to as primary oscillation. When the sensor experiences a rotation about an axis perpendicular to the primary oscillation or primary movement, the movement of the primary oscillation results in a Coriolis force proportional to the measurement quantity, i.e. the angular velocity. The Coriolis force induces a second oscillation orthogonal to the primary oscillation. This second oscillation orthogonal to the primary oscillation is referred to as secondary oscillation. The secondary oscillation, which is also termed detection oscillation, can be detected by several measuring methods, with the quantity detected serving as a measure for the rotational rate acting on the rotational rate gyroscope.
To generate the primary oscillation, thermal, piezoelectric, electrostatic and inductive methods are used among others, which are known in the art. For detecting the secondary oscillation, piezoelectric, piezoresistive and capacitive principles are state of the art.
Known micromechanical rotational rate gyroscopes are described in K. Funk, A. Shilp, M. Offenberg, B. Elsner and F. Lxc3xa4rmer, xe2x80x9cSurface Micromachining Resonant Silicon Structuresxe2x80x9d, The 8th International Conference on Solid-State Sensors and Actuators, Eurosensors IX, NEWS, pages 50 to 52. In particular, a known, quasi-rotating gyroscope described in that publication comprises a circular oscillator supported on a base so as to be rotatable in two directions. The oscillator of the known gyroscope is of disc-shaped configuration with respect to an x-y plane, with comb electrode configurations be attached on two opposite sides of the disc. A comb electrode configuration is used for driving the oscillating body and is composed of fixed comb electrodes and the comb electrodes of the oscillator engaging with the fixed comb electrodes. A similar comb electrode detection assembly consists of fixed comb electrodes engaging with corresponding comb electrodes attached to the primary oscillator. The comb electrode configuration on the input side serving for driving the oscillator and being also referred to as comb drive, is suitably connected to an excitation voltage, such that a first comb electrode configuration is fed with an a.c. voltage, whereas a second comb electrode configuration of the comb drive is fed with a second voltage phase-shifted by 180xc2x0 with respect to the first voltage. Due to the applied a.c. voltage, the oscillator is excited to perform a rotational oscillation about the z axis normal to the x-y plane. The oscillation of the oscillator in the x-y plane is the afore-mentioned primary oscillation.
When the known gyroscope is rotated about an y axis with a specific angular velocity, a Coriolis force acts on the oscillator that is proportional to the applied angular velocity about the y axis. This Coriolis force generates a rotational oscillation of the oscillator about the x axis. This rotational oscillation or periodic xe2x80x9ctiltingxe2x80x9d of the oscillator about the x axis can be measured capacitively by means of the two electrodes located underneath the gyroscope or sensor.
A disadvantage of this known structure consists in that the primary oscillation and the secondary oscillation, which is the oscillation of the oscillating body due the Coriolis force acting thereon, are carried out by one single oscillator supported by means of a two-axis joint in order to be able to perform the two mutually orthogonal oscillations. The two oscillation modes, i.e. the primary oscillation and the secondary oscillation, thus are not decoupled from each other, and this is why the intrinsic frequencies of primary and secondary oscillations cannot be balanced in exact manner independently of each other in order to obtain an as high as possible sensing accuracy of the rotational rate gyroscope. Furthermore, in case of the known gyroscope, the secondary oscillation has the effect that the comb electrode assembly for driving the oscillator is tilted, thereby affecting the primary oscillation by the secondary oscillation. This influence results in a primary oscillation that is not controlled in fully harmonic manner, which is a reaction to the retroactive effect of the secondary oscillation on the primary oscillation, i.e. a reaction to tilting of the comb drive for generating the primary oscillation.
Another known rotational rate gyroscope described in that publication comprises two mutually separate oscillatory masses which may be brought into opposite-phase oscillation by respective comb drives connected to one mass each by spring beams. The two masses are connected to each other by a spring beam arrangement and, due to suspension of the assembly of the two masses and the connecting webs of the masses, carry out a rotational oscillation in the x-y plane when the gyroscope is subjected to rotation about the z axis. Displacement of the assembly of the two masses and the spring beams mutually connecting the masses, in the direction of the y axis as a reaction to rotation of said assembly is detected capacitively by means of four comb electrode configurations.
Just as the first known gyroscope described, the second known gyroscope also comprises merely one single oscillator both for the primary oscillation and for the secondary oscillation, so that the two orthogonal oscillation modes are coupled with each other and the secondary oscillation generated by the Coriolis force may have a retroactive effect on the primary oscillation. This structure, too, thus permits no exact selective balancing of the intrinsic frequencies of the primary and secondary oscillations.
A further known oscillatory gyroscope is described in the article by P. Greiff et al., entitled xe2x80x9cSilicon Monolithic Micromechanical Gyroscopexe2x80x9d in the conference band of Transducers 1991, pages 966 to 968. This gyroscope is a double gimbal structure in the x-y plane, which is supported by torsion springs. A frame-shaped first oscillator structure surrounds a plate-shaped second oscillator structure. The second oscillator structure comprises an inertia element projecting in z-direction from the plane thereof. In operation, rotary excitation about the y axis of the first oscillator structure is transferred by torsion springs rigid in the direction of the first oscillation to the second oscillator structure. In the presence of an angular velocity about the z axis, a Coriolis force is generated in the y direction, which engages the projecting inertia element or gyro element in order to deflect the second oscillator structure about the x axis, whereby the second oscillator structure performs a Coriolis oscillation about the x axis orthogonal to the excitation oscillation, which is rendered possible by the torsion springs suspending the second oscillator structure on the first oscillator structure. The Coriolis force present with this gyroscope only in y direction does not result in movement of the remaining structure since the latter is fixedly held in the y direction. Only the gyro element projecting in y direction offers a point of application for the Coriolis force, so that this force can effect a measurable movement proportional to the forced rotation.
Although the first and second oscillations are decoupled from each other in this structure and no retroactive effect of the second oscillation on the excitation of the first oscillation takes place, a disadvantage resides in that the second oscillator structure cannot be made in planar manner due to the projecting gyro element. Upon manufacture of the gyroscope structure, the gyro element is formed by gold electroplating on the second oscillator structure. Such electroplating is not favorable for integration in a substantially planar monolithic manufacturing process, causing an increase in manufacturing time and manufacturing steps as well as rising costs for the gyroscope.
DE 44 28 405 A1 discloses a rotational rate gyroscope comprising an excitation actuation mechanism with comb drives and two oscillatory masses connected to each other via spring members so as to constitute an oscillatory system. The gyroscope comprises in particular a first oscillation structure that can be excited by means of comb drives so as to perform an excitation oscillation. Via connecting points, the excitation oscillation is transferred from the first oscillation structure to a second mass. Various springs and holding means connect the second mass to a central oscillatory mass, the springs having the effect of transferring the excitation oscillation from the second oscillatory mass to the central oscillatory mass and of causing the two oscillatory masses to oscillate with mutually opposite phases due to the excitation oscillation. Upon rotation of the gyroscope, a Coriolis force acts among other things on the central oscillatory mass, having the effect that said mass moves perpendicularly to the excitation oscillation. The Coriolis force acts also on the second mass, the latter experiencing a Coriolis oscillation opposite to the central mass, which is opposite for the reason that the two m asses have excitation oscillations of opposite phases.
DE 195 00 800 A1 discloses Coriolis rotational rate gyroscopes having two oscillatory masses which are mechanically coupled to each other and constitute an oscillating structure. The two oscillatory masses, on their opposite faces, each have two symmetrically arranged bending springs by means of which the oscillatory masses are mechanically coupled with each other using additional silicon webs. In a first category of Coriolis gyroscopes, the secondary oscillators are excited directly, without the use of a primary oscillator. In case of a second category of Coriolis gyroscopes, two oscillatory masses are excited by a primary oscillator to oscillate with opposite phases, with a web transferring the primary oscillation to the secondary oscillators the oscillation of which passes through a holding point connected to a substrate, as well as through springs attached to the substrate.
EP 0 634 629 A discloses an angular velocity sensor comprising a first oscillating member supported by a first supporting beam, and a second oscillating member connected to the first oscillating member by means of a second supporting beam. The firs t supporting beam, through which the first oscillating member is connected to the substrate, is provided in addition with a comb drive in order to cause the first oscillating member to oscillate in a first direction. The second supporting beam connecting the first oscillating member and the second oscillating member also permits oscillation in the first direction, whereby the excitation oscillation on the one hand is transferred to the second oscillating member and on the other hand is increased. Upon rotation of the angular velocity sensor about an axis normal to the first direction, a Coriolis force acts on the arrangement, which causes deflection of the second oscillating member in z direction. In doing so, the first oscillating member is held so as to be movable in the direction of the Coriolis force.
It is the object of the present invention to provide a rotational rate gyroscope which can be produced in economical manner and in which primary oscillation and secondary oscillation are largely decoupled.
This object is met by a rotational rate gyroscope for detecting rotation thereof about an axis of rotation, comprising: a base member; a primary oscillator adapted to be excited to perform a primary movement which is directed normal to the axis of rotation or about an axis normal to the axis of rotation, respectively; a secondary oscillator adapted to be caused, by a Coriolis force, to perform a secondary movement which is directed normal to the axis of rotation or about an axis normal to the axis of rotation, respectively, and normal to the primary movement, with major surfaces of the primary oscillator and the secondary oscillator extending substantially in the same plane and the movement of the primary oscillator and/or the movement of the secondary oscillator taking place in this plane; a first spring means constituting a primary oscillator suspension and holding the primary oscillator so as to be movable with respect to the base member; a second spring means separate from the first spring means and connecting the primary oscillator to the secondary oscillator and constituting a secondary oscillator suspension, wherein the primary oscillator suspension is designed such that it moves the primary oscillator in the direction of the primary movement, and that the secondary oscillator suspension is designed such that it transfers the primary movement to the secondary oscillator substantially in rigid manner; that it moves the secondary oscillator in the direction of the secondary movement; and that it substantially prevents a transfer of the secondary movement back to the primary oscillator.
The invention is based on the finding that decoupling of primary and secondary oscillations can be achieved by providing a primary oscillator held so as to be movable with respect to a base member by means of a primary oscillator suspension. A primary oscillation applied to the primary oscillator is transferred via a secondary oscillator suspension to a secondary oscillator, so that the secondary oscillator also performs the primary oscillation. A Coriolis force present due to rotation of the rotational rate gyroscope results in a secondary oscillation of the secondary oscillator orthogonal to the primary oscillation of the secondary oscillator which, by way of a suitable design of the secondary oscillator suspension, has no retroactive effect on the primary oscillator. The primary oscillator suspension, depending on the particular embodiment, may consist of suitably dimensioned spring beams (e.g. torsion springs or bending springs) being of such cross-section and geometric arrangement (e.g. diagonal struts, number etc.) that it provides a direction-dependent spring rigidity. This anisotropy of the rigidity of the suspension in principle can be guaranteed solely by the arrangement of the spring beams. The secondary oscillation thus has no retroactive effect on the primary oscillator, so that the excitation is not influenced by the quantity to be measured. By providing a secondary oscillator separate from the primary oscillator and due to the configurations of the primary oscillator suspension and the secondary oscillator suspension, which also is spatially separate from the primary oscillator suspension and has only preferably an anisotropic rigidity, the primary and secondary oscillations are decoupled from each other to the largest possible extent, so that both, the primary oscillation and the secondary oscillation can be balanced independently of each other.
A two-axis joint for an oscillator, as present in the prior art, which so to speak is concentrated in one spatial spot and which permits the mutually orthogonal primary and secondary oscillations of the sole oscillator, is converted in the rotational rate gyroscope according to the present invention to two mutually separate joints and oscillators, which on the one hand constitute the primary oscillator suspension and primary oscillator, respectively, and on the other hand constitute the secondary oscillator suspension and secondary oscillator, respectively. The provision of a second oscillator, i.e. the secondary oscillator, which is connected to the primary oscillator via the secondary oscillator suspension, permits the two oscillations to be decoupled. The primary oscillator is driven to perform a translational or rotational oscillation which via the secondary oscillator suspension is transmitted to the secondary oscillator. A Coriolis force acting due to rotation of the rotational rate gyroscope, by way of a suitable design of the primary oscillator suspension, however acts only on the secondary oscillator, and not on the primary oscillator, so that said excitation is not influenced by the quantity to be measured. Furthermore, the oscillation of the secondary oscillator due to the Coriolis force can be transferred only insignificantly to the movement of the primary oscillator by the secondary oscillator suspension. The rotational rate gyroscope according to the present invention thus indeed permits a transmission of the primary oscillation from the primary oscillator to the secondary oscillator, but no transmission of the secondary oscillation back to the primary oscillator.
Due to the construction of the vibratory gyroscope according to the present invention in such a manner that both the primary and the secondary oscillators extend substantially in the same plane, manufacture becomes simple since the vibratory gyroscope can be manufactured in fully compatible manner with known planar manufacturing techniques. Due to the fact that, in addition thereto, the primary oscillation and/or the secondary oscillation take place in the plane in which primary oscillator and secondary oscillator are formed as well, the Coriolis force always can act on the substantially planar secondary oscillator such that it can be excited to oscillate.